1. Field of the Invention
The present invention is directed to a luminescent storage screen having a stimulable phosphor for the latent storage of x-ray images.
2. Description of the Prior Art
Luminescent storage screens are known in the art wherein a latent x-ray image is stored using a stimulable phosphor, with read-out of the x-ray image being achieved by exciting the phosphor with radiation of a first wavelength (stimulating radiation), which causes the phosphor to emit radiation of a second wavelength, which is acquired by a detector. Such a luminescent storage screen is disclosed, for example, in European Application 0 174 875.
A luminescent storage screen of this type employed in an image pick-up device is described in U.S. Pat. No. 3,859,527. In such an x-ray diagnostics installation, a luminescent storage screen, consisting of a luminescing stimulable phosphor which is irradiated with x-rays, is used as a radiation-sensitive transducer. Electronic holes are generated in the stimulable phosphor by the incident radiant intensity, these holes being stored in traps having a higher energy level, so that the latent x-ray image is stored in the screen.
For read-out, the entire area or surface of this screen, used as a master, is caused to luminesce pixel-by-pixel by an additional radiation source, which may be a laser. Due to the stimulating radiation, the energy of the holes stored in the traps is boosted and they can fall back into lower energy levels, whereby the energy difference is radiated in the form of light quanta. The stimulable phosphor thereby emits light dependent on the energy stored in the phosphor. The light emitted as a result of this stimulation is detected and rendered visible, so that the x-ray image which was latently stored in the screen can be read out.
A problem in the read-out of such conventional screens is that the stimulable phosphor is not sufficiently transparent for the laser light. A minimum thickness of the stimulable phosphor is required to be able to achieve adequate x-ray quantum absorptions. In the case of a non-transparent, tightly compressed or sintered phosphor, the laser beam is so greatly attenuated by the phosphor that the penetration depth of the laser beam is too small. Because the energy is no longer adequate for boosting the holes to the energy level required for quantum emission, the information stored in the deeper levels cannot be read out.
The storage screen disclosed in the European Application 0 174 875 has phosphor grains which are applied on a substrate enveloped by a binder. The binder serves the purpose of fixing the phosphor grains. A light-transmissive carrier material is usually employed as the binder, which is transparent both for the exciting laser light and for the emitted luminescent light. A problem associated with screens of this type, however, is that the laser beam spreads increasingly with increasing penetration depth, due to scattering of the beam at the phosphor grains, so that the modulation transfer function of the overall system is degraded. A storage screen in binder technology also has poorer quantum x-ray quantum absorption, compared to a layer of comparable thickness of the stimulable phosphor.
It is preferable, however, to vapor-deposit the stimulable phosphor onto a carrier in a high vacuum, and to temper the phosphor in a protective gas atmosphere, or in the vacuum, or to compress the phosphor under vacuum and/or heating, as disclosed in European Application 0 369 049. It is also possible to reshape single crystals of stimulable phosphor to the large area required for medical diagnostics by compressing such crystals. The latter methods yield transparent stimulable phosphor panels. The advantage of the transparency is that the stimulating laser beam cannot be spread in the storage medium due to scattering at the grains of the material. Such spreading of the read-out beam, as noted above, considerably degrades the modulation transfer function of the overall system. Spreading of the laser beam upon transirradiation of the storage medium is greatly diminished by using a transparent stimulable phosphor manufactured, for example, by compressing the phosphor powder.
The powder pressing process, however, has three disadvantages.
First, particle boundaries between the powder particles can remain after pressing, these boundaries acting as scatter centers for the read-out radiation. The scattered read-out beam then also reads information from locations that are not correctly chronologically allocated to the read-out beam. The signal-to-noise ratio in the resultant image is thus degraded. The reason for the possible remainder of particle boundaries is that the powder particles can only be deformed by pressure when the pressure is coupled from one particle to the next, so that plastic deformation of the crystal occurs at that location. Because the particles touch at only a few points, it frequently occurs that a particle does not experience pressure from the suitable direction which would be required so that the plastic deformation is complete.
A second disadvantage is that many alkali halogenides experience a phase transition in the presence of high pressure. In the case of a RbBr stimulable phosphor doped with TlBr, this phase transition occurs at approximately 500 MPa. The RbBr stimulable phosphor at that point converts from a face-centered cubic structure (a NaCl-type structure) into a body-centered cubic structure (a CsCl-type structure). This transition takes place with a reduction in volume, and is reversible. When the pressure is removed after the pressing process, the crystal reverts into the NaCl-type structure. Because of the increase in volume associated with the re-conversion into the face-centered cubic structure, cracks and fissures may occur in the storage medium, which act as scatter centers for the read-out radiation.
A third disadvantage is that, due to the necessary coupling of the pressure from one powder particle to the next, the required pressure for making powders transparent by pressing is extremely high, greater than 500 MPa, and preferably 1000 MPa in the case RbBr, so that the manufacture of a large-area storage medium is extremely difficult.